A growing body of literature suggests that remediating aquatic ecosystems with selenium additions could be a solution to mercury contamination, yet elevated selenium concentrations can have toxicological effects on aquatic animals. University and USGS researchers searched existing literature to evaluate evidence supporting assertions of selenium benefits. Experimental evidence shows that selenium can mediate mercury toxicity, trophic transfer, and methylation in aquatic ecosystems. However, adverse effects following mercury and selenium co-exposure have been reported in studies with various taxa. Limited or ambiguous support indicates that scientific uncertainties remain regarding selenium mediation of mercury behavior and toxicity in abiotic and biotic compartments. More information is needed to provide a strong scientific basis for modifying current fish consumption advisories based on selenium-to-mercury ratios or for applying selenium amendments to remediate mercury-contaminated ecosystems. 

Gerson, J.R., Walters, D., Eagles-Smith, C.A., Bernhardt, E.S., Brandt, J.E., 2020, Do two wrongs make a right? Persistent uncertainties regarding environmental selenium-mercury interactions: Environmental Science and Technology, v. 54, no. 15, p. 9228–9234, https://dx.doi.org/10.1021/acs.est.0c01894